Double methane borylation

28 March 2016 - Coincidences do not exist

Interesting double paper header in this week's issue of the journal science, a methane borylation and a methane borylation! A University of Michigan team beat a University of Pennsylvania team by just 6 days! And as coincidences do not exist in chemical land this is worth investigating. Team Michigan (by the way 75% women compared to 0% women in team Penn) describes a methane borylation (link). Key inspiration: a 2005 paper by Hall/Hartwig on computations on methane borylation with pentamethylcyclopentadienyl ruthenium catalysts. Publications like these can sit around for 11 years but eventually they will get picked up. The experimental validation provided by the Michigan team looks like this: the reaction of methane in with bis(pinacolato)diboron in cyclohexane @150°C and @2500 kPa with 3 mole% of Bis(pentamethylcyclopentadienyl)ruthenium(II) (Cp*)2Ru will give a 99% yield of the mono borane adduct. The ratio of mono-substitution versus di-substitution is 10:1. Not a small feat as the researchers explain because although the C-H bonds in methane are the least sterically hindered, those in the boron reagent are more acidic and those in the solvent are most numerous. With a more realistic catalyst loading of 0.75% the yield drops to 52% but the selectivity increases.

And how did the Pennsylvania team (link) get their inspiration? They do not cite the 2005 Hall/Hartwig paper but a 2010 Hartwig review. They note that breaking the boron boron bond in diboron reagents provide an enthalpic driving force and also that many iridium catalysts are commercially available. Hence their optimised result: The reaction of methane with bis(pinacolato)diboron in cyclohexane @150°C and 3500 kPa catalysed with 0.5% cyclooctadiene iridium chloride dimer and DMPE gives a 52% yield. Exactly the same result as team Michigan but with much lower selectivity.

But since when do coincidences exist? One team member on the Penn team, Milton R. Smith III, happens to be a professor at the University of Michigan... So how well do Milton and team Michigan get along these days?